Power system reactors are often used in combination with resistors and capacitors to perform filtering functions, to control the inrush and outrush from capacitor banks, etc. In many of these applications the parallel combination of a reactor and a resistor appear. The purpose of this combination is to alter the native characteristics of the reactor at frequencies higher than the system frequency such that the combination presents a much lower quality factor to these frequencies and absorbs very large power at these frequencies.
The combination of a reactor and resistor in parallel is used for example in series with a capacitor to form a filter which presents a high impedance to the power frequency but a much lower impedance to a band of harmonic frequencies. Another arrangement is where the capacitor is also in parallel with the coil and resistor. This latter filter circuit presents a high impedance to a band of harmonic frequencies and a low impedance to the power frequency. In both cases, the resonant frequency is established primarily by the inductance and capacitance of the circuit and the bandwidth primarily by the resistance.
A third combination which consists of the foregoing arrangements in series results in a filter which presents a low impedance to two selected frequencies, these frequencies being established by the choice of the LC combinations for the two parts of the filter.
All three of the above combinations are often used to filter out harmonics generated by power semiconductor switching devices on power systems, for example, in DC to AC transformations and for controlling the reactor power flow in static compensator systems.
The combination of a reactor and a resistor in parallel is also used to control the inrush and outrush from large capacitor banks when these are switched in and out of power systems.
The resistors conventionally used in parallel with reactors are separate devices and in the case of outdoor installations must be housed in waterproof enclosures. The chief advantage of the separate resistor is the fact that the dissipation depends only on the voltage across the resistor (and therefore, the voltage across the reactor) and is independent of the frequency. The use of the separate parallel resistor for dissipation of large amounts of energy, however, is costly both in terms of the equipment itself and in terms of installation space required.
A filter choke capable of handling high power levels is disclosed in U.S. Pat. No. 3,808,562, issued Apr. 30, 1974 and includes a choke coil and an active resistance element connected in parallel with the choke coil. The active resistive element is magnetically neutral, neither generating a magnetic field to influence the choke coil nor is it noticeably influenced by the magnetic field of the choke coil.